1. Field of the Invention
The present invention relates to apparatuses for performing cipher communication in order to prevent illegal eavesdropping and interception by a third party, and more particularly, relates to a data transmitting apparatus, a data receiving apparatus, and a method executed thereby for performing data communication through selecting and setting a specific encoding/decoding (modulating/demodulating) method between a legitimate transmitter and a legitimate receiver.
2. Description of the Background Art
Conventionally, in order to perform communication between specific parties, there has been adopted a structure for realizing secret communication by sharing key information for encoding/decoding between transmitting and receiving ends, and by performing, based on the key information, an operation/inverse operation on information data (plain text) to be transmitted, in a mathematical manner. FIG. 11 shows a configuration of a conventional data communication apparatus based on the above-described structure.
In FIG. 11, the conventional data communication apparatus has a configuration in which a data transmitting apparatus 9001 and a data receiving apparatus 9002 are connected to each other via a transmission line 913. The data transmitting apparatus 9001 includes an encoding section 911 and a modulator section 912. The data receiving apparatus 9002 includes a demodulator section 914 and a decoding section 915.
In the data transmitting apparatus 9001, information data 90 and first key information 91 are inputted to the encoding section 911. The encoding section 911 encodes (encrypts), based on the first key information 91, the information data 90. The modulator section 912 converts the information data encrypted by the encoding section 911 into a modulated signal 94 in a predetermined modulation method and transmits the same to the transmission line 913.
In the data receiving apparatus 9002, the demodulator section 914 demodulates, in a predetermined demodulation method, the modulated signal 94 transmitted via the transmission line 913. To the decoding section 915, second key information 96 which has the same content as the first key information 91, which is shared with the encoding section 911, is inputted. The decoding section 915 decodes (decrypts) the modulated signal 94 in accordance with the second key information 96 and outputs the original information data 98.
Here, by using an eavesdropper's data receiving apparatus 9003, eavesdropping by a third party will be described. In FIG. 11, the eavesdropper's data receiving apparatus 9003 includes an eavesdropper's demodulator section 916 and an eavesdropper's decoding section 917. The eavesdropper's demodulator section 916 eavesdrops on the modulated signal 94 transmitted between the data transmitting apparatus 9001 and the data receiving apparatus 9002, and decodes the eavesdropped modulated signal 94 in a predetermined demodulation method. The eavesdropper's decoding section 917 attempts decoding of the demodulated information data, in accordance with third key information 99. Here, due to no key information sharing with the encoding section 911, the third key information 99 is different in content from the first key information 91. Therefore, the eavesdropper's decoding section 917 cannot accurately reproduce the original information data 90 inputted to the encoding section 911 even if the decoding is performed based on the third key information 99.
A mathematical encryption (or also referred to as a computational encryption or a software encryption) technique based on such mathematical operation may be applicable to an access system as described, for example, in Japanese Laid-Open Patent Publication No. 9-205420 (hereinafter referred to as patent document 1). That is, in a PON (Passive Optical Network) structure in which an optical signal transmitted from an optical transmitter is divided by an optical coupler, and distributed to optical receivers at a plurality of optical subscribers' houses, such optical signals that are not desired and aimed at other subscribers are inputted to each of the optical receivers. Therefore, information data for each of the subscribers is encrypted by using key information which is different by the subscribers, whereby it is possible to prevent a leakage/eavesdropping of mutual information and realize safe data communication.
However, in the case of the conventional data communication apparatus based on the mathematical encryption technique, even if the eavesdropper does not share the key information, it is theoretically possible for the eavesdropper to succeed in decryption, with respect to a cipher text (modulated signal or encrypted information data), by means of an operations using all possible combinations of key information (an all-possible attack), or by means of a special analysis algorithm. Particularly, improvement in the processing speeds of computers has been remarkable in recent years, and thus there is a problem in that if a new computer based on a novel principle such as a quantum computer is realized in the future, it is possible to eavesdrop on the cipher text easily within finite lengths of time.